1. Field
Various exemplary embodiments of the present invention relate to a memory system, and more particularly, to a read method of the memory system capable of improving reliability of data read.
2. Description of the Related Art
Semiconductor memory devices are generally classified into volatile memory devices and nonvolatile memory devices. A volatile memory device loses data stored therein when a power supply thereto is interrupted, whereas a nonvolatile memory device retains data stored therein even when a power supply thereto is interrupted. Examples of the nonvolatile memory devices are a ROM (Read Only Memory), an EEPROM (Electrically Erasable ROM), and so forth. A flash memory device which is Introduced as a flash EEPROM is different in structure and operation from a conventional EEPROM. The flash memory device performs an electric erase operation by unit of block and a program operation by unit of bit.
When a memory device performs a program/erase cycle a predetermined number of times or performs the program operation on the memory block and the number of times the program/erase cycle is performed is over a predetermined number of times, the distribution of the threshold voltages of the memory block may change. The change of the threshold voltages of the flash memory device may deteriorate the reliability of a read data. Therefore, the number of error bits should be minimized by reading data through changing the optimal read voltage. That is, the read voltage is set using a method of providing a read-retry table to a controller and sequentially changing the read voltage included in the read-retry table during a read operation, a method of analysing the gradient of the threshold voltage in the threshold voltage distribution, and a method of analysing threshold voltage at which a number of memory cells is minimized between the read voltage sections.
However, the methods use threshold voltage distribution and thus there is a low possibility of setting the optimal read voltage.